The type of motion detector here considered comprises a device for sensing incident radiation, usually infrared rays, which may be emitted by an associated source elsewhere in the space under surveillance and whose interruption by an intruder sets off an alarm. Alternatively, such a device can be used to sense heat waves from the body of the intruding person.
In either case, the motion detector is designed to respond only to changes in the intensity of the incident radiation. It is therefore desirable to subdivide the surveyed spaced into a number of relatively narrow fields of view in which a given disturbance causes a more pronounced intensity variation. Reference in this connection may be made, for example, to U.S. Pat. No. 3,923,382 according to which a multifaceted concave mirror focuses radiation from respective fields of view onto a single sensing element. See also U.S. Pat. Nos. 3,958,118 and 4,268,752 as well as German printed specification (Auslegeschrift) No. 26 45 040 and German published application (Offenlegungsschrift) No. 21 03 909.
In U.S. Pat. No. 4,087,688 as well as in my prior German application No. 28 36 462 (published Mar. 6, 1980) there is further disclosed a somewhat simplified focusing system for the purpose referred to in which beams incident at certain angles are focused by a refractive or reflective optical system onto a single sensor via a prismatic or cylindrical housing with reflecting internal wall surfaces extending parallel to the optical axis. The angles of incidence for which this is true are given by arctannd/f where d is the inner diameter or width of the housing, n is an integer (0, 1, 2 etc.) and f is the distance between the sensor and a point of intersection of all the beams on the axis at the housing entrance. The angular separation of the beams decreases with increasing values of n while their internally reflected rays strike the receiving area of the sensor at progressively larger angles to the optical axis. Thus, detection of motion becomes less certain with increasing distance from that axis.